Certain metals such as electrolytic tinplate have been difficult to cover with adhesive resin systems. In particular, electrolytic tinplate which is relatively inert and therefore excellent for manufacture of sanitary cans for containing certain foods without additional inside coatings, can not be decorated with an ultraviolet light activated ink or some conventional inks. While it is known that organic titanates are useful for reducing the drying time of printing inks because the titanates catalyze the crosslinking of certain unsaturated resins and drying oils it was not known until the present invention to use titanate and ultraviolet treatment to condition the surface of an organic lubricant over electrolytic tinplate (ETP) such that subsequently applied ultraviolet activated inks could firmly adhere.
Currently, the industry uses solvent type inks containing alcohols or esters which must be driven off by baking, some of these however will not adhere to ETP. Such curing is wasteful of energy of petrochemicals and deleterious to the environment. The solvents tend to dissolve or dilute the lubricant or contaminants on the plate surface, thus allowing better contact between the ink solids and the metal surface, and some of the lubricant is volatilized during the thermal bake. Thus, the adhesion of some thermal inks on the tinplate is not affected by the lubricants or contaminants. The problem with the use of ultraviolet activation for high speed ink curing (by solidification of the monomeric ink resin) is that such curing is instantaneous and traps the mobile surface constituents and the lubricant and thus adhesion to the metal surface is poor. The commonly used lubricants are acetyl tributyl citrate, dioctyl sebacate or butyl stearate all of which are relatively nonoxidizing liquids. When they are applied to the metals at a rate of 0.17-0.28 grams per 62,720 square inches of surface, a coating is formed slightly in excess of a monomolecular layer which acts to increase mobility and prevent abrasion during handling and shipment. Bare tinplate surfaces usually contain electrolytically deposited chromium or its oxides which passivates the tin surface to prevent tin oxide growth in warm, humid environments. In the past such metal surfaces were frequently flame or corona treated to reduce or oxidize organics, however, these techniques are slow and energy intensive and require capital investment for expensive line equipment and space in the plant.